Another possible option is something that is used at my university for mailing list owners. Once a year I get an e-mail saying that I have to "renew" it (i.e. basically, acknowledge that I still exist and have a need for it) within some window of time or it gets disabled.
As above, there were plans to introduce a cron job into the CVS module that "disabled" cvs accounts (and an appropriate e-mail was to be sent) that hadn't witnessed any activity for X months. Any projects that were associated with disabled accounts were to be deemed abandoned. This tackles the issue more from a "target absconding maintainers" rather than "target unmaintained projects" perspective. This also keeps things uncomplicated and minimises drupal.org related code in the module. -K